This thesis deals with buck converters in 0.18m HV-CMOS technology. Research in the field of monolithic DC-DC converters with emphasis on approaches which lead to full integration of the power devices and regulation loop on the chip, is performed. The two-stage converter with output power of several watts and with switching transistors implemented by NMOS transistors, potentially enables reaching better performances compared to the standard solutions, but at the same time, introduces new difficulties and challenges, which are all addressed in this work. The focus in the thesis will be on the careful calculations and dimensioning of the transistors and circuits, their design and layout, implementation, and finally on the simulation and measurement results of the converters. The work furthermore proposes a novel integrated bootstrap circuit, which provides the voltage overdrive for the gate of the High-Side (HS) switching transistor without using an external diode nor additional voltage supply, and a novel integrated current sensing circuit with included comparator function, eliminating the need for a separate comparator circuit, and concurrently making the whole regulation loop more precise and robust.